a) Field of the Invention
The present invention on relates to a compact zoom lens system which has a high vari-focal ratio.
b) Description of the Prior Art
To obtain a compact zoom lens system which has a high vari-focal ratio, it is effective to compose a lens system, in order from the object side, of a first lens unit having positive refractive power, a second lens unit having negative refractive power, a third lens unit having positive refractive power and a fourth lens unit having positive refractive power, and configure the lens system so as to change a magnification by moving the lens units.
As conventional examples of such a type of zoom lens system, there are known lens systems disclosed as fourth and sixth embodiments of Japanese Patent Kokai Publication No. Sho 58-78114, lens systems disclosed as first through fourth embodiments of Japanese Patent Kokai Publication No. Sho 58-224323, a lens system disclosed as a third embodiment of Japanese Patent Kokai Publication No. Sho 59-164517, lens systems disclosed as first through fifth embodiments of Japanese Patent Kokai Publication No. Sho 63-70819, a lens system disclosed as a second embodiment of Japanese Patent Kokai Publication No. Hei 3-29912, a lens system disclosed as a first embodiment of Japanese Patent Kokai Publication No. Hei 4-208911, a lens system disclosed as a second embodiment of Japanese Patent Kokai Publication No. Hei 5-173070, a lens system disclosed as a third embodiment of Japanese Patent Kokai Publication No. Hei 6-75167, a lens system disclosed as a first embodiment of Japanese Patent Kokai Publication No. Hei 8-43736, lens systems disclosed as first through third embodiments of Japanese Patent Kokai Publication No. Hei 6-337354, lens systems disclosed as first through eighth embodiments of U.S. Patent Publication No. 4256381 and a lens system disclosed as a fourth embodiment of U.S. Patent Publication No. 4299454.
However, the zoom lens systems quoted as the conventional examples have vari-focal ratios lower than 5 and cannot be said as zoom lens systems which have sufficiently high vari focal ratios, and it is difficult to further enhance the vari focal ratios of these zoom lens systems.
Furthermore, lens systems disclosed as first through fourth embodiments of Japanese Patent Kokai Publication No. Sho 62-270910 have high vari-focal ratios which are higher than 5, but a field angle of 64.degree. at a wide position which is not large and it is difficult to further enlarge the field angle.
Furthermore, a zoom lens system disclosed by Japanese Patent Kokai Publication No. Hei 8-86963 has a high vari-focal ratio which is 6 or higher, but uses an aperture stop disposed in an airspace as narrow as 0.03 mm to 0.21 mm which is varied for changing a magnification and reserved between a final lens element of a second lens unit and a first lens element of a third lens unit. This airspace must be at least 0.4 mm in an actual product. In other words, the lens elements may be brought into contact with the aperture stop dependently on variations of parts. Even if the zoom lens system can be assembled into the product, the lens system is hardly assembled, thereby constituting a cause for an enhanced manufacturing cost.
Furthermore, a zoom lens system disclosed by Japanese Patent Kokai Publication No. Hei 9-5629 has a high vari-focal ratio which is not lower than 6, first through third embodiments of this conventional example cause remarkable variations of image surfaces by changing magnification and have optical performance which is not favorable. Moreover, a zoom lens system preferred as a fourth embodiment of this conventional example exhibits favorable optical performance at wide, standard and tele positions, but unbalances spherical aberration, curvature of field and distortion at an intermediate focal length which is located on a side of the tele position from the standard position and cannot correct these aberrations at the same time. In addition, a zoom lens system preferred as a fifth embodiment of this conventional example produces remarkable distortion on the side of the tele position.